1. Field of the Invention
The present invention relates to combustible gas detectors or sensors and in particular to calibration means for combustible gas sensors.
2. General Background
A variety of industrial operations such as pulp mills, refineries, sewerage and waste water plants and the oil and gas industry constantly face the risk of the presence or production of combustible gases such as hydrocarbons. As an aid to increase safety in such areas, sensors have been developed which are sensitive to the presence of such gases. Upon sensing a pre-set level of the gas being monitored, the sensor provides an electric signal which sets off an alarm system to warn personnel of the unsafe condition. However, these sensors must be exposed to the gas on a regular basis to ensure that the sensor is properly calibrated. This helps prevent annoying and costly false alarms and failure to alarm when the gas reaches a dangerous level. However, this calibration operation has involved the necessary use of scaffolding or ladders to reach sensors located in remote locations which are difficult to reach or hazardous. The known art has presented a variety of approaches to the situation.
U.S. Pat. No. 4,305,724 entitled "Combustible Gas Detection System" discloses the use of an active sensor maintained at a controlled temperature in respect of a reference sensor and which measures combustible gases by means of a bridge-sampling system and a variable duty cycle system.
U.S Pat. No. 4,476,706 entitled "Remote Calibration System" discloses calibration of combustible gas detectors by signals applied to a sealed, explosion-proof sensor assembly to effect a remote calibration sequence.
Japanese Pat. No. JA57-128840 discloses a calibration device having a gas ported pressure sensor.
U.S. Pat. Nos. 4,555,930 entitled "Digital Gas Sensing System," 4,279,142 entitled "Technique For In Situ Calibration Of A Gas Detector," No. 4,534,204 entitled "Calibration Method For Ultra Sensitive Noble Gas Leak Detector," No. 4,553,423 entitled "Gas Doser," No. 4,124,475 entitled "Hydrogen Sulfide Monitoring System," No. 4,260,950 entitled "Automatic Portable PH Meter And Method With Calibration Receptacle," No. 4,432,224 entitled "Hydrogen Sulfide Measuring Systems And The Like," and No. 4,489,590 entitled "Method And Appartus For Gas Detector Calibration" all disclose combustible gas detection systems which are representative of the art.
Although a variety of systems are shown in the art, none these systems provide a means for calibrating systems through the use of calibration ports which do not require the complete replacement of the expensive sensors and housings. Replacing and discarding of entire sensors and housings is inefficent and expensive. Thus, there exists a need for providing means for calibrating sensors without the necessity of replacing the entire sensor and housing.